Yeri stands thirty first and writes the front jer, the second of the two reduced vowels of Old Church Slavonic. It carries no numeral value.
Like its back counterpart it was a real vowel in the earliest texts, short and weak, written wherever it was spoken. Its second function was to say that the consonant before it was softened, and when the vowel itself fell silent in the sound change described on the entry for yer, that second function was all that remained.
The letter therefore became a diacritic in the shape of a letter. Modern Russian calls it the soft sign, and it is one of the few letters in any alphabet whose entire job is to modify the letter before it while representing no sound of its own. Ukrainian, Belarusian and Bulgarian keep it too, with the same function and slightly different rules.
That transformation is worth stating plainly, because the modern letter is often described as though it had always been a mark of softness. It had not. It was a vowel that happened to imply softness, and it survived because the implication turned out to be more useful than the vowel. The alphabet that came out of the ninth century did not contain a single letter of this kind, and neither did the language it was made for.